Non-invasive methods are useful and popular to measure breathing movements and respiratory effort. Respiratory Inductive Plethysmography (RIP) is one such method, which includes the use of respiratory bands to measure respiratory effort related areal changes. RIP technology includes a measurement of an inductance of a conductive belt or belts that encircles a respiratory region of a subject.
The signal amplitude received from the respiratory effort belts depends on both the shape of the subject and the placement of the belts. To create a respiration volume signal by summing the signal of the respiratory effort belts, one must use correct weighting constants for the measured belt signals to transform each signal correctly into a volume signal before summing them together. Further, to perform a quantitative calibration, the signals of the respiratory effort belts must be measured simultaneously with a quantitative reference measure. Known methods therefore require quantitative equipment for respiratory volume measure, such as a spirometer, body-box or similar ways to measure respiratory volume accurately during the calibration.
Due to the complexity added with using reference respiratory volume equipment and the fact that the weighting constants are subject to change over time with belt and body movements, it would simplify the measurement of respiratory efforts considerably if there were a method available that would evaluate weighting constants without the need of special quantitative equipment for reference measures.
Statistical measures of RIP during normal breathing to evaluate weighting constants may be used for respiratory analysis and sleep diagnostics. However, the calculation of a calibration factor will change if the belts move or the subject changes position. To maintain accuracy, recalibration is needed after such movements and changes, which requires a few minutes of normal, non-obstructive breathing. This can be difficult with a sleeping subject, especially with subject's suffering from sleep disordered breathing.
A method for calculating and calibrating the respiratory signals in a more continuous fashion without the need for quantitative equipment would be advantageous.